1. Field of the Invention
This invention relates to a film drive device for use in cameras.
2. Description of the Related Art
Recent developments in motorized cameras have provided the impetus for the proposition of a capability of controlling coordination of the windup and rewind systems. The prior known coordinating technique employed a changeover mechanism such as a clutch positioned in the middle of a windup transmission system and arranged to be able to transfer the driving torque of an electric motor that used to work as the drive source for motorized winding-up to a rewind transmission system. Because of only one motor in use, all what was needed to coordinate the winding-up with the rewinding was to control the on-off operation of the changeover mechanism. In the motorized camera of this kind, for switching it from the windup to rewind mode, there are two occasions, one of which occurs after all film has been exposed, and the other of which arises when the film-rewind release button is manipulated. In the latter case, an event that the operative position of the changeover mechanism is changed at a time when the film is not used up can be even encountered.
Another type of motorized camera which is different from the above-described one in that two motors, respectively for the windup motor and the rewind motor, has been also proposed.
In any of the conventional types of motorized cameras, however, the coordinating means operates always in the same manner regardless of whether the transition from the windup to the rewind mode takes place automatically as all the frames of film have been exposed, or is carried out manually by the rewind switch. Such a feature might have no problem if it is applied to, for example, the leaf shutter camera of the type in which the motor is one in number and the operation of a planetary clutch as the coordinating means is controlled by changing the direction of rotation of the motor in such a manner that the driving torque of the motor is transferred from the windup transmission system to the rewind transmission system, since nothing more than merely to change over the planetary clutch is needed.
However, if there is either one of the additional features that the windup transmission system is associated with a charge mechanism for the reflex mirror and the shutter as in the single lens reflex camera, and that, from the standpoint of high price cameras, the means for detecting when no more fresh area of film can be fed is of elaborate design, the sequence of operations beginning with the detecting one and terminating at the start of rewind of the film becomes complicated. For a motorized single lens reflex camera having both of these two additional features, therefore, it is desired to vary the sequence of operations to a simpler one with the limitation of the number of operations to a minimum depending on the situation, so that electrical energy of the battery is prevented from being wastefully consumed, and, as that sequence advances faster, the photographer can manage his camera quickly and easily.
Meanwhile, the film windup transmission system of the above-described motorized single lens reflex camera is made to perform a high speed transportation of the film with a relatively low speed reduction in order to increase the number of film cartridges to be processed for a pack of batteries, leaving the number of batteries in the pack as an electrical power source not to be too much increased. Therefore, when the actual voltage of the electrical power source only somewhat lowers, or when the load of pulling the film out from the cartridge only slightly increases from the normal level, the transportation of the film is caused to stop, despite that all the frames of film have not been wound up yet. In this case, according to the prior art, an erroneous information representing the detection of the film end was given forth.